Technical Field
The present disclosure relates to a semiconductor integrated device for detection of the UV-index and to a related calibration system and method. The present description will make explicit reference, without this implying any loss of generality, to use of the device incorporated in a portable electronic apparatus, such as for example a smartphone, a tablet, a PDA (Personal Data Assistant), a digital audio player, a photographic or video camera, or the like, or else in a wearable electronic apparatus, such as for example a smart electronic bracelet or watch, or the like.
Description of the Related Art
Exposure to sources of natural light (sunlight) or artificial light involves assumption of a certain dose of ultraviolet (UV) radiation.
In a known way, ultraviolet radiation covers the portion of the electromagnetic spectrum with wavelengths between 100 nm and 400 nm and mainly divided into: UVA radiation (in the 315-400 nm range); UVB radiation (in the 280-315 nm range); and UVC radiation (in the 100-280 nm range).
In general, the degree of penetration of UV rays and thus the danger for human beings, increases as the wavelength decreases and, consequently, as the frequency increases.
The universal index of solar UV radiation, referred to more simply as UV-index (or UVI—Ultra-Violet Index), provides an indication of the level of UV radiation coming from a source. Typically, the UV-index refers to solar radiation that reaches the Earth's surface, but may likewise refer to any sources of ultraviolet radiation.
The UV-index has been conceived in the perspective of increasing the awareness of the population on the risks of an excessive exposure to UV radiation, in particular deriving from solar radiation and has been developed in the context of a co-operation between the World Health Organization, the United Nations Environment Programme (UNEP), the World Meteorological Organization and the International Commission on Non-Ionizing Radiation Protection (ICNIRP).
It is known, in fact, that UV radiation is a common cause of numerous health problems, amongst which: burns up to skin cancer; damage to eyesight up to the risk of cataract; immunosuppression.
The values of the UV-index are usually grouped into exposure categories, associated to which is a color scale indicating danger for humans, as summed up in the table below.
UV-indexExposure levelConventional color <2LowGreen3-5ModerateYellow6-8HighOrange 8-10Very HighRed>11ExtremePurple
In greater detail, the UV-index has been formulated to weight the various wavelengths of the spectrum, taking into account the biological effects proper to each frequency. In particular, it is a measurement of the capacity for UV radiation to induce an erythematous reaction and is obtained by weighting each frequency of UV radiation via the so-called “erythemal-action spectrum”.
The UV-index (dimensionless parameter) is given by the following expression:
                              U          ⁢                                          ⁢          V          ⁢                                          ⁢          I                ≅                  k          ⁢                                                    ∫                                  250                  ⁢                                                                          ⁢                  nm                                                            400                ⁢                                                                  ⁢                nm                                      ⁢                                          E                λ                            ⁢                                                S                  er                                ⁡                                  (                  λ                  )                                            ⁢                              ⅆ                λ                                                                        [        1        ]            where Eλ is the spectral irradiance associated with UV radiation (i.e., the power of the radiation incident upon a surface per unit area), expressed in the unit of measurement W/(m2·nm); k is a constant equal to 40 m2/W; and Ser(λ) is the aforementioned erythemal-action spectrum.
In particular, the plot of the erythemal-action spectrum, as defined by CIE (Commission Internationale de l'Éclairage), using the method described by McKinlay, A. F. and Diffey, B. L., is illustrated in FIG. 1 and is given by the following expression:
            S      er        ⁡          (      λ      )        ≅      {                            1                                                    250              ⁢                                                          ⁢              nm                        ≤            λ            ≤                          298              ⁢                                                          ⁢              nm                                                                        10            ⁢                                                  ⁢                          exp              ⁡                              [                                  0.094                  ·                                      (                                          298                      -                      λ                                        )                                                  ]                                                                                        298              ⁢                                                          ⁢              nm                        ≤            λ            ≤                          328              ⁢                                                          ⁢              nm                                                                        10            ⁢                                                  ⁢                          exp              ⁡                              [                                  0.015                  ·                                      (                                          139                      -                      λ                                        )                                                  ]                                                                                        328              ⁢                                                          ⁢              nm                        ≤            λ            ≤                          400              ⁢                                                          ⁢              nm                                                            0                                      λ            ≥                          400              ⁢                                                          ⁢              nm                                          
In practice, the erythemal-action spectrum Ser(λ) defines a weighting function for the energy associated with UV radiation, on the basis of the range of wavelengths.
FIG. 2 shows, by way of example, the curve of the spectral irradiance Eλ for solar radiation, measured at sea level and on a sunny day, as defined by the ASTM G173-03 standard.
Appearing once again in FIG. 2 is the plot of the erythemal-action spectrum Ser(λ) and the regions associated with UVB and UVA radiation are further highlighted. As may be noted, UVC radiation associated with solar radiation is absorbed by the Earth's atmosphere.
A wide range of detection devices, which supply a measurement of the UV-index, are known today.
In particular, scientific laboratory devices, for example including spectrometers, enable an accurate measurement of UV-index to be obtained by measuring the spectral contribution of UV radiation for each wavelength and then by computing, by post-processing, the value of the UV-index applying the above Eq. [1].
Non-scientific devices also exist, which provide measurements with a greater degree of approximation and are typically based upon the use of a photodetector, in particular a photodiode operating in the ultraviolet range.
In order to improve the precision of measurement, a filtering system has to be associated with the photodetector, typically including a quartz filter and a Teflon diffuser so that the frequency response of the photodetector will approximate the weighting plot of the erythemal-action spectrum Ser(λ).
By way of example, FIG. 3 shows the optical response of a UV photodiode to which a quartz filter is associated (solid line), superimposed on the plot of the erythemal-action spectrum Ser(λ) (dashed line).
The above detection devices of a known type are not suited, however, to integration in portable or wearable apparatuses.
In this regard, the tendency is in fact known to incorporate new detection functions within modern portable or wearable electronic apparatuses to provide the user with an increasing amount of information.
It is likewise known that integration of detection devices in portable/wearable electronic apparatuses must meet stringent requirements in terms of costs, dimensions and levels of consumption of electrical energy.
The aforesaid detection devices do not meet these requirements, given that they are typically too costly and complex to produce. Further, they may have dimensions that are not compatible with integration in portable or wearable electronic apparatuses.